1. Field of the Invention
This invention relates to a highly coherent laser source with extremely narrow spectral linewidth, and more specifically to an actively-stabilized single-frequency Brillouin fiber ring laser (with spectral linewidth measured in the Hertz range) that is pumped with a high-power single-frequency fast-tuned laser.
2. Description of the Related Art
Many applications such as coherent optical communications, coherent ladar detection, and microwave photonics require highly coherent laser sources with narrow spectral linewidth. Design and performance of such optical systems strongly depend on coherence properties of the laser sources used. Diode-pumped single-frequency solid-state lasers (including fiber lasers) are the most well-known highly coherent laser sources that have a spectral linewidth ranging from hundreds of kHz to a few kHz.
A single-frequency Brillouin fiber ring laser is another type of highly coherent light source, which is based on stimulated Brillouin scattering nonlinear optical process in optical fiber. The nonlinear interactions between laser optical fields and an acoustic wave result in Brillouin scattering process in an optical medium (fiber). The laser field (pump field) generates an acoustic wave through the process of electrostriction. The acoustic wave in turn modulates the refraction index of the medium. This pump-induced index grating scatters the pump light through Bragg diffraction. Because of the Doppler shift associated with a grating moving at the acoustic velocity, the back-scattered light, which is also called as Stokes radiation, is downshifted in frequency by vB=2 nVa/λ, where Va is the acoustic velocity in the fiber. When the pump light propagates in the medium, Brillouin gain at the shifted frequency can be established in the medium.
The Brillouin gain in optical fibers can be used to make lasers by placing the fiber inside a cavity. Both the ring-cavity and the Fabry-Perot cavity have been used for making Brillouin lasers, each having its own advantages. Brillouin fiber lasers consisting of a Fabry-Perot cavity exhibit features that are quite different from those using a ring-cavity configuration. The difference arises from the simultaneous presence of the forward and backward propagating components associated with the pump and Stokes waves in a Fabry-Perot cavity. Higher-order Stokes and anti-Stokes waves are generated through cascaded stimulated Brillouin scattering (SBS) and four-wave-mixing process. This is a way to generate multi spectral lines (frequency comb).
Most Brillouin fiber lasers use a ring cavity to avoid generation of multiple Stokes lines through the cascade SBS. The first demonstration of a cw Brillouin fiber laser using an argon-ion laser and ring-cavity configuration was reported in 1976. [Appl. Phys. Lett. 28 (1976) 608]. The performance of a Brillouin fiber ring laser depends on the fiber length used to make the cavity. For short fibers (˜10-40 m), the ring laser can operate stably in a single longitudinal mode (i.e., single-frequency) with extremely narrow linewidth. In contrast, a Brillouin ring laser with long fiber (>hundreds meters) operates in multiple longitudinal modes, and the number of modes increases with fiber length. The output of such long lasers can become periodic, even chaotic under some conditions. They can also exhibit mode-locking behavior under other conditions.
Due to their extremely narrow linewidth, single-frequency Brillouin fiber ring lasers pumped with a single-frequency pump laser have attracted significant interest for decades. Experiments have demonstrated that free-running spectral linewidth of the Stokes radiation generated from single-frequency Brillouin fiber ring lasers, which could potentially be only a few Hz that corresponds to a coherence length of the laser equal to tens thousands kilometers, can be several orders of magnitude narrower than that of the single-frequency pump beam used to generate stimulated Brillouin scattering in the cavity of Brillouin fiber ring laser. [S. P. Smith, F. Zarinetchi, and S. Ezekiel, “Narrow-linewidth stimulated Brillouin fiber laser and applications,” Opt. Lett. 16 (1991) 393.] and [J Boschung, L. Thevenaz, and P. A. Robert, “High-Accuracy Measurement of the linewidth of a Brillouin Fiber Ring Laser,” Electron. Lett. 30 (1994), p. 1488.]. In practice, however, stable single-frequency operation of Brillouin fiber ring lasers with extremely narrow linewidth is hard to realize and also is not practically useful if without active stabilization.
Brillouin fiber ring lasers (U.S. Pat. Nos. 4,107,628, 4,530,097 4,780,876, and 5,323,415) are typically lossy, unstable devices, in which no any active stabilization was used. In addition, these devices included a lot of free-space optics either for the Brillouin fiber ring cavity or for their bulk pump laser sources, which make the devices suffer from very poor stability and reliability.
In some other publications ([1]. Optics Letters, 6 (1981) 398. [2]. Electronics Letters, 25 (1989) 260. [3]. Journal of Lightwave Technology, 21 (2003) 546. [4]. Optics Letters, 28 (2003) 1888.), an active stabilization technique was mentioned. In all these prior publications, however, the active stabilization was achieved by piezo-electrically stretching part of the fiber in the Brillouin fiber ring cavity so that one cavity mode of the Brillouin ring laser is kept in resonance with its pump laser light. With this configuration of active stabilization, an auto-tracking feedback loop is applied to the Brillouin ring cavity via a PZT actuator to control the ring cavity length. Thus, the laser frequency of the Brillouin fiber ring laser output followed any frequency fluctuation of its pump laser, which usually has a much wider spectral linewidth and worse frequency fluctuation than those of the Brillouin fiber laser itself. As a result of the modulation and feedback loop, the stabilized Brillouin fiber ring lasers suffer from fast frequency modulation. As a result, the linewidth of the Brillouin fiber laser is no longer extremely narrow, it even could be wider than the narrow linewidth of its pump laser (see reference Optics Letters, 28 (2003) 1888.)